If they're not whole fillets but smaller pieces I would brine for 8-12 hours/overnight. I'll go 24-hours for a dry rub...less for brine. If you go 16-24 hours you won't ruin them...they'll just be a bit saltier and a bit drier for which you can compensate by placing a pan of water in the smoker during the smoking process.
After removing the pieces from the brine rinse them under cold/cool water removing herbs, spices, etc. Then pat the fillets dry with paper towels and place them on a rack so air can circulate under/around the fillets. A fan gently blowing over the fillets will speed the drying process. You'll want to let them rest in a cool place for an hour or so or until a slight film/sheen comes over the fillets. Then in to the smoker. Smoking is more effective if no moisture is on the meat/fish.
My go to wood for all things salmon, smoking or grilling, is alder. Having said that I'd love to try sggoat's orange wood suggestion! But any other wood is just not to my liking for salmon/trout.
Pecan OTOH is my favorite for smoking pork; however I do like using a wide variety of hardwoods including fruitwood for smoking meat. But you need to experiment, over time, and learn what you personally like best for the type of wood for the type of meat. Its fun combining different woods for different effect and flavor.
Not too much science with smoking but more art form! I recommend keeping a journal with each item you smoke. Brine/rub strength/recipes; amount (in ounces) of smoking wood along with the wood type and chunk size; temperature(s) you smoked the meat as well as time. And of course tasting notes.
Spraying/rubbing the clean metal grates with oil will help keep meat/fish/etc., from sticking as well as making clean up a bit easier.
Speaking of metal grates: Mine originally came with chrome plated steel which peeled in short order. Later they went to stainless steel which are available...and they work great along with finer mesh grates for jerky, small veggies, etc.
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